SINEUPs are antisense long non-coding RNAs that enhance translation of overlapping sense mRNAs through the activity of two domains: a SINEB2 sequence UP-regulating translation (Effector Domain, ED) and an antisense region providing target specificity (Binding Domain, BD). In this study, we demonstrate that the invSINEB2 sequence from the natural SINEUP AS Uchl1 RNA is an Internal Ribosomal Entry Site (IRES) when acting in cis and that known viral and cellular IRES sequences can act as Effector Domain in synthetic SINEUPs. To identify natural IRES-containing, non-coding RNAs with SINEUP-like activity, we focused on circular RNAs showing that the non-coding circ5533, transcribed from the c-myc locus, enhances endogenous protein expression of its target PX Domain Containing Serine/Threonine Kinase Like (Pxk) by increasing mRNA association to polysomes. In summary, this study shows that natural and synthetic SINEUPs include linear and circular transcripts with an embedded IRES sequence as ED.